1. Field of the Invention (Technical Field)
The present invention relates to high-sensitivity detection of contaminants in gases by optical techniques and to telecommunications applications by optical techniques generally implemented with wavelength tunable lasers.
2. Background Art
Diode lasers have become increasingly important for optical detection of gases (trace gas detection). Typically, high sensitivity detection is achieved with diode lasers by rapidly modulating the laser wavelength across an absorption feature of the target species. By rapidly modulating the laser wavelength, laser intensity noise is dramatically reduced. However, a drawback of using diode lasers for gas sensing applications is that they operate over a very limited wavelength range. Typically, only one species can be detected with a given laser. The output wavelength range of a diode laser can be expanded using an external cavity configuration. With such a configuration, multiple species detection is possible. However, external cavity diode lasers (ECDL) cannot be wavelength modulated at more than a few kHz. This inability to provide rapid wavelength modulation limits achievable gas detection sensitivity.
The present invention uses an external cavity design that overcomes the low modulation frequency limitations of present external cavity lasers. Furthermore, in contrast to present external cavity laser designs, the design described herein is simple, inexpensive to implement and rugged. The present invention combines the stability and tunability of an ECDL with the wavelength agility of a diode laser.
A successful commercial ECDL is produced by New Focus, Inc., and a similar device is offered by Newport Corporation. These ECDLs are based on the Littman-Metcalf grazing incidence design. M. G. Littman, et al., Appl. Opt. 17, 2224 (1978). Both instruments employ mechanical movement of a cavity feedback mirror. The maximum wavelength modulation frequency is limited to 2 kHz by the need to move the mirror. Such low modulation frequencies are less effective at reducing the laser xe2x80x9cexcessxe2x80x9d noise that is often the limiting noise source in wavelength modulation absorption measurements of trace gas concentrations. Because of the high dispersion employed in the Littman-Metcalf ECDL design, it is not possible to modulate the laser wavelength by modulating the diode laser injection current or temperature.
The New Focus and Newport ECDLs are designed to be general laboratory spectroscopic optical sources and they are well suited to the application. The lasers exhibit extended wavelength tuning ranges without mode hops (where a mode hop is a sudden, discontinuous change in laser output wavelength) and only minor variation in laser output power. They are capable of 2 kHz wavelength modulation at any wavelength within their nominal tuning ranges. Thus, the lasers are designed for obtaining high-resolution spectra of gaseous molecules over a broad wavelength range. This capacity is necessary for obtaining spectra of molecules for the first time or performing survey scans. Unfortunately, operation as an all-purpose tunable spectroscopic source prevents these commercial instruments from achieving the high-sensitivity required for trace gas detection. Furthermore, their cost is prohibitive in most sensing applications.
The present invention retains the broad wavelength tuning of the Littman-Metcalf design and can achieve the high frequency wavelength modulations that are useful for trace gas detection. The differences between the present invention and previous ECDL designs are substantial and will become apparent through further description of the design. The wavelength modulation frequency of the present invention is limited only by the injection current modulation response of the diode laser used as the gain element. Thus, wavelength modulation frequencies in the GHz regime are possible. In addition, the present invention retains the broad wavelength tuning range of commercial instruments. The present invention may not work as well for laboratory survey spectroscopy because it does not tune without mode hopping and the output amplitude is not constant. Nevertheless, the present invention is superior for trace gas detection and the reduced complexity with increased capability results in an ECDL that is much lower cost than present commercial instruments.
Commercial ECDL manufacturers have expended significant effort to provide continuous single mode tuning in a single mechanical movement. To meet this requirement the cavity length must change concomitantly with the angular selection of the cavity feedback mirror. This capability is the basis of New Focus"" U.S. Pat. Nos. 5,319,668 and 5,995,521. The present invention obviates the requirement for continuous single mode tuning by specifically allowing longitudinal mode hops that are controlled by the diode laser gain element injection current and temperature. Mode hops work to advantage because they are controlled so as to occur predictably and reproducibly. In addition, the present invention mode hops wavelengths successively in a single direction at a time as determined by the laser injection current or diode laser temperature.
A significant portion of optical sources used in telecommunications are continuous wave (cw) single frequency diode lasers. Direct amplitude modulation of these optical sources with injection current is not often utilized in high frequency and long haul applications. Instead, the information encoding on these optical sources is typically added downstream of the laser using electro-optic modulators. The present invention improves upon single frequency continuous wave ECDLs, making them suitable as optical sources for telecommunications.
Typical diode lasers used in telecommunications, particularly those used for dense wavelength division multiplexing (DWDM) applications, are based on distributed feedback (DFB) structures. The DFB structure requires post-growth processing and results in devices an order-of-magnitude more expensive than Fabry-Perot based structures. Although DFB lasers have some temperature and current tuning capability, tuning ranges are limited relative to ECDL designs. An individual DFB laser is suitable for only one DWDM channel. The present invention combines the less expensive Fabry-Perot laser structure with other inexpensive optical components to allow operation at any one of many DWDM channels. The overall cost of the ECDL is about the same as a DFB laser that is limited to operation at only one DWDM channel.
The ECDL of the present invention, then, is well suited as a back-up device for DWDM transmitters. If a primary DFB-driven channel fails, the ECDL can take over until the channel can be repaired. Because the ECDL can operate on many DWDM channels, it can act as a temporary replacement for many DFB lasers. Alternatively, with the present advancement towards dynamically reconfigurable DWDM transmitters, a suite of the ECDLs of the present invention would be used as primary optical sources. Each ECDL could be configured to operate on any one of many DWDM channels so that channels could be added or dropped based on the real-time bandwidth requirements.
Other telecommunications applications utilizing tunable optical sources of the invention also improve on the state of the art. Examples include test and measurement of telecommunications components in the field and during research and development.
The present invention is of an external cavity diode laser and method of generating laser light, comprising: generating light from a Fabry-Perot diode laser source; collimating light from the source with an intracavity optical element; reflecting light via a feedback mirror; and employing a diffraction grating to receive light from the optical element, diffract received light to the mirror in a non-zero order, receive reflected light from the mirror, and direct reflected light back towards the optical element and Fabry-Perot diode laser to complete an external cavity, the diffraction grating additionally directing a portion of the received light from the optical element toward a target. In the preferred embodiment, the Fabry-Perot diode laser lacks an anti-reflection coating or has an anti-reflection coating that does not interfere with residual diode laser Fabry-Perot modes providing sufficient differential gain when tuned by injection current or temperature to allow single frequency operation and single longitudinal mode-hopping within a pass band of the grating. A multiple pass optical configuration can be employed to provide a sufficiently narrow longitudinal mode spacing. The mirror preferably reflects a first order diffraction from the grating, provides wavelength selectivity to the light directed toward the target, and provides a sufficiently low dispersion to allow wavelength tuning by longitudinal mode hopping within a grating pass band through interaction of the grating pass band with diode laser Fabry-Perot modes where the interaction simultaneously ensures operation over successive single longitudinal modes. The wavelength of the light directed toward the target may additionally be tuned by varying a diode gain element injection current of the diode laser source or by varying the temperature of the diode laser source. Wavelength modulation of the light directed toward the target may be accomplished by injection current modulation or by diode laser temperature modulation. In an optical system, the target may be an optical fiber light injection apparatus.
Objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.